The present invention relates to an improved electrostatic chuck.
In the plasma processing of articles such as semiconductor wafers, a common problem is the coupling of electrical energy to the article being processed. Typically, electromagnetic coupling of RF energy into the source region of a plasma chamber is employed to generate and maintain a high electron density plasma having a low particle energy. In addition, RF bias energy is usually capacitively coupled in the plasma via the article being processed to increase and control the energy of ions impinging on the article.
In a typical high density plasma reactor, the driving point RF bias impedance presented by the plasma is very low. To achieve uniform ion energy and flux to the article being processed (typically essential for etching or other plasma processes), uniform coupling of RF bias energy through the article being processed to the plasma is required. The article, typically a semiconductor wafer, being processed normally is held against some kind of chuck and RF bias energy is applied to the chuck. What is desired is a constant plasma sheath voltage across the surface of the wafer being processed.
The degree to which such a uniform plasma sheath voltage can be achieved is a function not only of the plasma density uniformity as generated by the plasma source, but is also a function of the impedance per unit area of the plasma sheath adjacent to the wafer, the impedance per unit area of the wafer, the impedance per unit area of any gap between the wafer and the chuck and the impedance per unit area of the chuck.
Besides electrical coupling, the chuck should be tightly thermally coupled to the wafer being processed. Typically the temperature of the article is a process parameter to be controlled and this normally means removing heat from or adding heat to the wafer during processing. Heat transfer in a low pressure or vacuum environment such as that used for plasma processing is generally poor. Some means of providing for adequate heat transfer between the wafer being processed and adjacent surfaces is usually necessary.
Gas is typically introduced between the wafer and chuck to enhance thermal contact and heat transfer from the wafer to the chuck. The gas pressure required is a function of the heat load imposed by the plasma, the desired maximum wafer temperature, the temperature at which the chuck can be maintained (such as with liquid cooling), the choice of cooling gas and the wafer/gas and gas/chuck accommodation coefficients (measures of how effectively heat is transferred between a gas and a surface). For biased high density plasma applications, helium gas is used as the cooling gas and the gas pressure required is typically in the 5 to 30 torr range.
For "low pressure" plasma processes (those operating in millitorr pressure range), some means must be provided to allow a significantly higher pressure in the region between the wafer and chuck with respect to the ambient pressure in the process chamber. In addition, a leak of cooling gas into the process environment may produce undesirable results.
Prior art mechanical chucks have proven inadequate for use in the plasma processing of articles such as semiconductor wafers. Mechanically clamped chucks suffer from shortcomings such as mismatches in curvatures between the wafer and the chuck, resulting in a variable gap between such surfaces and consequent non-uniform electrical and thermal coupling.
Electrostatic chucks have successfully overcome the non-uniform coupling associated with mechanical chucks. Electrostatic chucks employ the attractive coulomb force between oppositely charged surfaces to clamp together a wafer and a chuck. In principle, with an electrostatic chuck, the force between wafer and chuck is uniform for a flat article and flat chuck. The electrostatic force between the wafer and the chuck is proportional to the square of the voltage between them, proportional to the relative permittivity of the dielectric medium (typically a ceramic such as alumina) separating them (assuming conductivity is negligible) and inversely proportional with the square of the distance between them. Typically for biased-wafer high density plasma processing application (such as SiO.sub.2 etching) a cooling gas is required to improve the heat transfer between the wafer and the chuck to acceptable levels.
Introduction of gas cooling between a wafer and a chuck, while required to achieve adequate heat transfer, can cause problems with prior art electrostatic chucks when used in biased-wafer high density plasma applications. In particular, the requirement of introducing cooling gas in the region between an wafer and a chuck requires that some discontinuity be introduced in the chuck surface, typically some type of hole(s) through the chuck to a gas passage behind the chuck surface. The introduction of any discontinuity in the chuck surface distorts the electric field in the vicinity of the discontinuity, making arc breakdown and glow discharge breakdown of the cooling gas more probable. With DC bias applied between an wafer and a chuck, and RF bias applied to the chuck, gas breakdown becomes probable with prior art electrostatic chucks such as described in U.S. Pat. Nos. 4,565,601 and 4,771,730.
The problems of cooling gas breakdown are addressed by the electrostatic chuck described in U.S. Pat. No. 5,350,479, to Collins et al., assigned to Applied Materials, Inc. and incorporated in it entirety herein by reference. With reference to FIGS. 1-4, an electrostatic chuck 10a as described in the foregoing patent comprises an aluminum pedestal 14a within which a plurality of axially oriented lift pin holes 66a and gas conduits 48a having upper ends 50a are defined. The conduits 48a typically have a diameter of about 0.3 inch (8 mm). The conduits 48a preferably do not extend to the upper surface 40a of the pedestal 14a, but are separated from the surface by a thin intervening layer 60a of aluminum. A dielectric layer 44a of alumina or alumina/titania is plasma-sprayed over the upper surface 40a of the pedestal 14a of the electrostatic chuck 10a and ground to achieve a thickness of about 0.010 inch (0.25 mm).
As best shown in FIG. 3, a plurality of perforations 58a having diameters at least an order of magnitude less than the diameter of conduit 48a are formed through the dielectric layer 44a and the intervening aluminum layer 60a into the upper ends 50a of conduits 48a, preferably by laser drilling. These perforations permit transport of cooling gas from each conduit 48a to the surface of the dielectric layer 44a. In order to distribute the cooling gas from the perforations 58a over the upper face 54a of the dielectric layer 44a, a pattern 18a of one or more gas distribution grooves 55a, 56a is formed in the upper face 54a. These grooves are typically produced by a laser machining operation, and extend over the majority of the surface of the upper face 54a of the dielectric layer 44a. Thus, when a semiconductor substrate or other article is placed on the electrostatic chuck 10a, cooling gas flows through the conduits 48a and perforations 58a into the distribution grooves 55a, 56a, thus cooling the substrate or article.
The foregoing electrostatic chuck can be subjected to high power RF fields and high density plasmas immediately above the substrate chucked thereto without breakdown of the cooling gas due to arcing or glow discharge. However, a number of new problems arise in connection with the foregoing electrostatic chuck.
The dielectric layer 44a typically is formed by a plasma-spraying (also known as flame-spraying) process. Plasma-sprayed dielectric layers, typically finished to thickness in the range from about 50 to 500 .mu.m, have high dielectric strength in vacuum (&gt;1.5.times.10.sup.5 V/cm).
Plasma-sprayed dielectric layers are, however, somewhat porous, having a typical void fraction of about 1-10% more usually about 1-2%. The pores of such a dielectric layer form a complex three-dimensionally connected network throughout the dielectric layer. The porosity of such a layer can be characterized as "microporosity" or "macroporosity", or as a combination of both types of porosity. "Microporosity" occurs as a result of the shrinkage of the grains of the plasma-sprayed dielectric material upon cooling. "Macroporosity" occurs primarily as a result of inadequate melting of grains of the dielectric material during the plasma-spraying process.
The network of pores can trap contaminants. More importantly, the network of pores forms a conduction path for any mobile charge which is present in the dielectric layer. For example, plasma-sprayed dielectric layers tend to hydrate upon exposure to air. Excessive mobile charge may be generated due to this hydration. Charge may also be injected into the layer if the layer is exposed to plasma and an electric field is applied between the electrostatic chuck and the plasma, which is required to establish the electrostatic force by which the substrate is chucked.
The presence of excessive mobile charge in the plasma-sprayed dielectric layer 44a cn significantly increase the leakage current through the layer at the high electric fields (on the order of 10.sup.5 V/cm) typically employed with electrostatic chucks used for plasma processing of semiconductor wafers. The increased leakage current in turn degrades the electrostatic force.
A need has existed for an electrostatic chuck in which the leakage current density is substantially reduced or eliminated even in the presence of moisture or injected charge. A need has also existed for a simple method of making such an improved electrostatic chuck.